This application relates to a design for small volume electrochemical test strips suitable for amperometric determination of analytes in a liquid test sample. Disposable, single-use electrochemical test strips are commonly employed in the determination of analytes, particularly by diabetes in the determination of blood glucose levels. Advancements in design of these strips have frequently focused on the ability to use smaller samples, since smaller blood samples can be obtained with less pain. Examples of such test strips can be seen, for example for U.S. Pat. Nos. 5,437,999, 5,650,062, 5,700,695, 6,484,046, and 6,942,770 and Applications Nos. US2004/0225230, and US2006/0169599A1.
In known electrochemical sensors for determination of glucose the reactions depicted in FIGS. 1A and 1B may be used. Glucose present in the sample is oxidized in reaction 11 by an enzyme, such as glucose oxidase to form gluconic acid (or gluconolactone) and reduced enzyme. The reduced enzyme is regenerated to its active oxidized form by interaction 12 with oxidized mediator, for example ferricyanide. When there is an appropriate potential difference between the working and counter electrodes, the resulting reduced mediator is converted 13 to oxidized mediator at the working electrode, with concurrent oxidation of reduced mediator at the counter electrode 14. In addition the reduced mediator may diffuse 15 from the counter electrode and when it reaches the working electrode it can be converted 17 to oxidized mediator and diffuse 16 to the counter electrode complete the cycle.
FIG. 2 shows two exemplary current traces in sample cells according to the prior art, in which the working and counter electrodes are disposed in a closely spaced facing (sandwich) arrangement (dotted line) and a more openly spaced side-by-side arrangement (solid line). The mediator is freely diffusible between the two electrodes in both cases. The x-axis (time) starts when the measurement potential is supplied. An initial current spike 21 is observed in both traces that results from the charging of the double layer on, and consuming the mediator 13 close to, the portion of the electrode surface covered at the time of the measurement potential is first applied. Thereafter, there is a decline in current 22 because of the smaller flux of mediator arriving at the working electrode, resulting from the depletion of mediator in the vicinity of the electrode. In the solid trace this persists to longer times and lower currents 23 because of continued depletion of the mediator. In the dotted line a limiting current 24 is reached, which is caused by a stable flux of reduced mediator being generated at the nearby counter electrode 14 and diffusing 15 to the working electrode. This is balanced by a flux 16 of oxidized mediator going the other way.
Determination of the analyte concentration in solution can be made at various points along the current traces. When the electrodes are in a closely spaced facing arrangement this includes at the peak value 21, the plateau level 24, or during the decrease 22 in between; the plateau current 24 has a simple linear relationship with analyte concentration and the estimate of the current can be improved by averaging data in this region over a time period. When the electrodes are side-by-side the analyte concentration can be determined from the data at the peak value 21 or in the decrease 22, 24.
To use data from the decrease 22, 24 it is possible to recalculate the current data as the inverse square of current. The results of such a calculation on the data from both traces of FIG. 2 are shown in FIG. 3, with the minimna 31 corresponding to the peaks 21, the initial straight slopes 32 corresponding to the curved decline in current 22, and the continuation of the curving decline 23 being a continuation 33 of the initial straight slope 32 for the side-by-side geometry. The plateau for the sandwich geometry 24 also manifests as a plateau 34.
The results with the sandwich geometry of FIGS. 2 and 3 require a sample cell with electrodes that are sufficiently close that flux of reduced mediator 15 being generated 14 at the counter electrode arrives rapidly and stabilizes rapidly at the working electrode 13 during the course of data collection. The transition between the working electrode being unaffected by the counter electrode and being in a steady state with the flux from the counter electrode produces the curve between the straight parts 32 and 34 in FIG. 3. To minimize the time required for the test, it is desirable to decrease the time required for diffusion to occur, and for a steady state current to be established. Commonly assigned US Patent Publication No. 2005/0258036, which is incorporated herein by reference discloses an approach to this problem, adapted for use in the context of facing electrodes. The electrodes are in close proximity, which allows the system to reach the stable plateau quickly. This favours small sample chambers and hence small sample sizes.
However, this approach is not well-suited to electrodes disposed on the same substrate, i.e., to side-by-side electrodes. The results with the side-by-side geometry of FIGS. 2 and 3 require a sample cell that is sufficiently large that flux of reduced mediator being generated at the counter electrode 14 does not arrive at the working electrode during the course of measurement. This favours large sample chambers and hence large sample sizes. Decreased sample sizes can be achieved simply by placing the electrodes closer together, but placing the electrodes closer together means flux 15 from the counter electrode arrives at the working electrode and generates an additional signal 17, causing the region 33 to bend, reducing the accuracy of the estimate of concentration from the slope. However, the side-by side geometry means that a steady state will not be set up as rapidly as in the sandwich geometry and so the bending will last a long time before reaching a stable plateau where more reliable data will be available to estimate the analyte concentration.
The present invention provides a simple approach to decreasing the time required to arrive at a steady state current for an electrochemical test strip with side-by-side electrodes and increasing the accuracy of the estimated concentration of the analyte that can be achieved without increasing the complexity of the manufacturing process.